Robert, earl of Gloucester, (born c. 1090—died Oct. 31, 1147) chief supporter of the royal claimant Matilda during her war with King Stephen of England (reigned 1135–54).

The illegitimate son of King Henry I of England (reigned 1100–35), he was made Earl of Gloucester in 1122. After the death of Henry I and usurpation of power by Stephen (December 1135), Gloucester became the leader of the party loyal to Matilda, his half sister, who had been designated heir to the throne by Henry I. He took Matilda to England in September 1139 and at the head of her forces won from Stephen most of western England and southern Wales. In February 1141 he captured Stephen at Lincoln and imprisoned him in Bristol. Later that year Gloucester was captured at Winchester, Hampshire, and exchanged for the king. He continued to be the mainstay of Matilda’s cause until his death. The 12th-century chroniclers considered Gloucester an able and sagacious leader.

No one was enthusiastic about accepting Matilda as queen, especially as her husband, Geoffrey of Anjou, was actually at war with Henry at the time of his death. Robert, Earl of Gloucester, one of Henry’s many illegitimate sons, was an impressive candidate for the throne, as were Henry’s nephews, Theobald and Stephen of Blois. The outcome of the struggle in 1135 was unexpected: while Theobald,...